Stabilized laser source

ABSTRACT

The invention relates to the stabilization of semiconductor laser diode sources as they are extensively used in the field of optical communication. Such lasers are mostly employed as so-called pump laser sources for fiber amplifiers, e.g. Erbium-doped fiber amplifiers, and are designed to provide a narrow-bandwidth optical radiation with a stable power output in a given frequency band. To improve the stability of such laser sources compared to prior art designs, a plurality of “external” cavities is provided. In the commonly employed optical fibers for conducting the laser beam, these cavities may be formed by a plurality of appropriately designed Bragg gratings. However, the cavities may as well be formed by other means reflecting a given amount of the energy back to the laser in a desired frequency band, thus effecting the stabilization of the laser&#39;s intensity and frequency.

FIELD OF THE INVENTION

The invention relates to the stabilization of a laser, specifically asemiconductor diode laser of the type commonly used in opto-electronics,mostly as so-called pump lasers for fiber amplifiers in the field ofoptical communication, e.g. for Erbium-doped fiber amplifiers. Suchlasers are designed to provide a narrow-bandwidth optical radiation witha stable power output in a given frequency band. In particular, theinvention concerns an improved design of the external cavity exhibitinga significantly improved stability compared to prior art designs.

BACKGROUND AND PRIOR ART

Semiconductor lasers of the type mentioned above have, for example,become important components in the technology of optical communication,particularly because such lasers can be used for amplifying opticalsignals immediately by optical means. This allows to design all-opticalfiber communication systems, avoiding any complicated conversion of thesignals to be transmitted which improves speed as well as reliabilitywithin such systems.

In one kind of optical fiber communication systems, the lasers are usedfor pumping Erbium-doped fiber amplifiers, so-called EDFAs, which havebeen described in various patents and publications known to the personskilled in the art. An example of some technical significance are 980 nmlasers with a power output of 150 mW or more, which wavelength matchesthe 980 nm Erbium absorption line and thus achieves a low-noiseamplification. InGaAs lasers have been found to serve this purpose welland are used today in significant numbers. However, the invention is inno way limited to InGaAs lasers.

There are other examples of lasers of other wavelengths and materialsfor which the present invention is applicable. Generally, laser diodepump sources used in fiber amplifier applications are working in singletransverse mode for efficient coupling into single-mode fibers and aremostly multiple longitudinal mode lasers, i.e. Fabry-Perot (or FP)lasers. Three main types are typically being used for Erbium amplifiers,corresponding to the absorption wavelengths of Erbium: InGaAsP andmultiquantum-well InGaAs lasers at 1480 nm; strained quantum-well InGaAslasers at 980 nm; and GaAlAs lasers at 820 nm.

One of the problems occurring when using semiconductor lasers for theabove purpose is their wavelength and power output instability which,though small, still affects the amplification sufficiently to look for asolution to the problem. This problem is already addressed in Erdogan etal. U.S. Pat. No. 5,563,732, entitled “Laser Pumping of ErbiumAmplifier”, which describes the stabilization of a pump laser of thetype described above by use of a Bragg grating in front of the laser.This grating forms an external cavity with the laser. The laserbandwidth is broadened and stabilized by the reflection from thegrating. It is believed that the laser operation in so-called“coherence-collapse” is obtained by providing sufficient externaloptical feedback, here from a fiber Bragg grating within the opticalfiber into which the laser light is usually coupled. This grating isformed inside the guided-mode region of the optical fiber at a certaindistance from the laser. Such a fiber Bragg grating is a periodicstructure of refractive index variations in or near the guided-modeportion of the optical fiber, which variations are reflecting light of acertain wavelength propagating along the fiber. The grating'speak-reflectivities and reflection bandwidths determine the amount oflight reflected back into the laser.

Ventrudo et al. U.S. Pat. No. 5,715,263, entitled“Fibre-grating-stabilized Diode Laser” describes an essentially similarapproach for providing a stabilized laser, showing a design by which thelaser light is coupled to the fiber by focussing it through a fiberlens. Again, a fiber Bragg grating is provided in the fiber's guidedmode portion, reflecting part of the incoming light back through thelens to the laser. To summarize, when positioning a fiber Bragg gratingbeyond the coherence length of the laser and when the laser gain peak isnot too far from the Bragg grating's center wavelength, it is understoodthat the laser in coherence collapse operation is forced to operatewithin the optical bandwidth of the grating and thus iswavelength-stabilized. Additionally, low-frequency power fluctuationsseem to decrease by the effect of induced high-frequency multi-modeoperation.

In general, the above-described prior art devices must have a length ofthe external cavity, i.e. the optical fiber, somewhere at least between0.5 and 1 m, to definitely assure coherence collapse laser operation.For some even up to 2 m long optical fibers are required. This ratherlong fiber determines the size of the laser source and makes itcomparatively bulky.

Some types of semiconductor lasers, especially others than those in theabove mentioned patents, e.g. lasers having a narrow spectral gainwidth, are seen to exhibit instability at certain operating conditions,in particular undesirable switching from multi-mode to single-modeoperation within the grating bandwidth. This mode switching (coherencecollapse occurs in both cases) results in a fluctuation of the effectivelaser output which in turn produces noise, thereby negatively affectingor actually disturbing the amplification process. The mode-switchingproblem is aggravated by new generations of semiconductor laser diodeshaving at least twice as much output power than the lasers in theVentrudo or Erdogan patent and the desire of the industry to havewavelength stabilization for all possible operating conditions of alaser.

Other techniques have been proposed to correct fiber amplifier outputpower fluctuations, e.g. active methods to control the variations in thefiber amplifier output by feedback of an electric signal effecting acorrection of the laser power. A further solution is an electronicdithering circuitry forcing the laser to operate multimode, described byHeidemann et al. in U.S. Pat. No. 5,297,154, entitled “Fiber-OpticAmplifier with Feedback-Insensitive Pump Laser”. However, the need foractive components for these solutions add complexity and cost.

For a quite different purpose, Fischer et al. describe in“High-dimensional Chaotic Dynamics of an External Cavity SemiconductorLaser”, Phys. Review Letters, Vol. 73, No. 16, October 1994, pp.2188-2191, an experimental laser setup with an external T-shaped cavitycomprising a beam splatter and high reflecting gold mirrors at each ofthe two ends of the cavity's two arms. Though this layout shows anexternal two-cavity arrangement, it is absolutely unsuitable for thepurpose of the present invention, since the lengths chosen for the armsof the cavity and the reflectivities of the laser's exit facet and theabove-mentioned gold mirrors are selected to avoid the coherencecollapse just the opposite of the present invention, where coherencecollapse is a prerequisite.

Also in a very different field, Wang Xianghyang et al. disclose a“Theoretical and Experimental Study on the Fabrication of Double FiberBragg Gratings” in the journal Optical Fiber Technology: Materials,Devices and Systems, Vol. 3, No. 2, pp. 189-193. Double gratings areprovided at the same location within the fiber and this “chirped”grating is said to widen the transmission spectrum of the fiber. Again,this publication does nowhere address the problem that the inventionintends to solve.

Thus, it is the main object of the invention to devise a simple andreliable laser source layout, especially for pump lasers in opticalfiber communication systems, that provides a stable output under alloperating conditions. A specific object is to avoid the detrimental modeswitching of the laser, even for a laser output power of more than 150mW, and thus increase the stability of the output of high power lasersources. Output stability shall be achieved for high optical powerhaving reduced low frequency noise, wavelength stability and high sidelobe suppression outside the fiber Bragg grating bandwidths.

A further object is to allow maximum flexibility for choosing the lasersparameters without running into stability problems.

A still further object is to avoid any further complexity and keep thenumber of additional components of the laser source within a laserpumped optical amplifier to a minimum.

A particular object is to create a stabilized laser source of reducedsize by using a significantly shortened external cavity region.

THE INVENTION

In brief, to solve the problems addressed above, the present inventiondoes not use a single grating or cavity in front of the laser, but aplurality of appropriately arranged cavities. These cavities arepreferably arranged in series, but can also be arranged in parallel. Ifthe lengths of the cavities, their reflectivities, and their peakwavelengths are chosen accordingly, the laser is forced to operatemultimode under all or practically all operating conditions.

Whereas one single grating is known to act as a wavelength broadeningand stabilization element, it is understood that multiple cavitiesaccording to the invention impose a useful destabilization only withinthe compound bandwidth of all gratings high enough to force the laserinto multimode operation. In a way, this phenomenon may be named as a“photonic dither” with a similar effect as an electronic dither, but bymeans of passive components only.

Another advantage of the invention is that the total length of theexternal cavities can be reduced to less than the length of the priorart designs. This provides for smaller laser sources.

One preferred embodiment according to the invention has a firstreflector in front of a semiconductor laser diode forming a first cavityand, at an optimized distance, a second reflector in front of the firstone, forming a second cavity between the first and the second reflectorwithin the optical fiber. The peak wavelength of the second reflectormay be chosen close, but not necessarily identical to that of the firstreflector. Also, a certain offset of the peak wavelengths and/orbandwidths of the two reflectors may improve performance. Further, anyone or both cavities may be shorter than the coherence length of thelaser diode.

Preferably, the phase relationship between the two reflectors is chosensuch that the resulting waves or fields are—statisticallyseen—practically out of phase.

The reflectors can be of any kind suitable for the desired purpose; theyare preferably provided as Bragg gratings within the optical fiber,which simplifies their making and avoids the need to have any parts orcomponents added.

A preferred method for providing the desired plurality of cavities is toestablish them by simultaneously producing the desired Bragg gratingswithin the optical fiber. This keeps the additional efforts to fabricatethe additional cavities at a minimum and, at the same time, provides forclose tolerances of the desired layout.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, various embodiments of the invention shall bedescribed by reference to the drawings, in which:

FIG. 1 shows the layout of a first embodiment using fiber Bragggratings;

FIG. 2 depicts the layout of a second embodiment using other reflectorsinstead;

FIG. 3 shows a third embodiment with cavities on both ends of the laser;

FIG. 4 is a fourth embodiment with frontal and lateral cavities;

FIG. 5 is a graph of a laser's optical bandwidth at −20 dB down from themaximum stabilized by a single fiber Bragg grating according to therelated art;

FIG. 6 is a graph showing the improvement when a second cavity isimplemented according to the invention (here using a second fiber Bragggrating).

FIG. 1 shows the basic layout of a first embodiment according to theinvention. A semiconductor diode laser 1, here an InGaAs quantum welllaser, produces a laser light beam 3 that is emitted predominantly fromthe laser front facet 2. This beam is coupled into a suitable opticalfiber 5 via a fiberlens 4, focusing the beam onto the input end of thefiber 5. Within the optical fiber 5, a first fiber Bragg grating 6 a isarranged in a distance L1 from the laser 1. In a distance L2 from thisfirst grating 6 a, a second fiber Bragg grating 6 b is provided. Thecontrolled and now incoherent—as described above—exiting light beam 7leaves the optical fiber 5 and is fed into a fiber amplifier, e.g. anErbium-doped fiber amplifier not shown here.

The semiconductor laser is usually of a type that emits confined lightin a single transversal and lateral mode, but has several longitudinalmodes due to the Fabry-Perot cavity formed between the front and rearfacet. If the laser facet reflectivity has a value as low as 10⁻⁵, thelaser cavity extends essentially into the fiber with the fiber Bragggrating defining the end facet. In this case, the laser operates morelikely in coherence with the fiber Bragg grating. Thus, a higher frontfacet reflectivity of the laser typically on the order of 4% is desiredto ensure coherence-collapse. On the other hand, if the reflectivity ischosen too high, the optical output power is decreased. The efficiencyof the light coupled from the laser into the fiber through the fiberlens can be about 70% in production and approaching 85% in the laband/or for specially designed lasers. Hence, the efficiency of fiberBragg grating back reflection into the laser is then given by thesquared coupling efficiency times the Bragg grating reflectivity.Typically, more than 90% of the light passes the Bragg grating whereasthe rest is reflected back into the laser, if it is the first grating,or passing through another fiber Bragg grating with a part of itreflected back again.

If the wavelength of the free-running laser without backreflection,roughly corresponding to the laser gain peak, is too far from the fiberBragg grating peak, the laser may fall off the locking to the Bragggrating. A 20 nm wavelength range can be typically locked to the fiberBragg grating peak, denoted as the so called capture range. The gratingbandwidth is determined by the needs for pump wavelength channelseparation in EDFAs with a typical maximum limit of 2 nm. From amanufacturing viewpoint, the full-width half-maximum bandwidths can bechosen between 0.4 and 0.8 nm for a certain peak reflectivity. Alaser-to-grating distance of >50 cm ensures coherence collapse, but thisdistance can be smaller if several gratings (cavities) are used. Thewell known fiber Bragg grating fabrication is based on exposure to UVradiation periodically along a piece of the optical fiber, as describede.g. by Raman Kashyap in “Fiber Bragg Gratings”, Academic Press, 1999.

The reflectivity of the multiple gratings is an optimization versusoutput power. The effective, or compound reflectivity given by allgratings can be in the same range as the laser facet reflectivity. Awavelength overlap is also necessary to establish another cavity. Aconcrete example with two gratings, each with 3% reflectivity, (i.e.2×3% reflectivity) and 0.6 nm bandwidth at the same peak wavelengthgives excellent results. The function of having multiple cavities isgiven by the roundtrip time the portion of the backreflected light needsto get back into the laser cavity. It can be seen as a “photonic dither”with frequencies corresponding to the roundtrip times through thevarious cavities. A working example has a laser-grating distance L1 of 1m and a grating-grating distance L2 of 10 cm, yielding roundtripfrequencies of 100 MHz and 1 GHz, respectively. Distortions at differentfrequencies determined by the roundtrip time (length) of the multiplecavities forces the laser to become multimode. The light leaving thelast fiber Bragg grating entering the path to the EDFA shows minimal lowfrequency noise with typically 90% of the light confined to thebandwidth determined by the grating. Some noise at high frequenciesexists due to mode beating, but does not interfere with the slowlyreacting ions of the subsequent Erbium-doped amplifier.

A different method to get distortion by light reflected back into thelaser is to have different grating wavelengths, where the first gratingacts as the master grating, locking the pump wavelength, and the secondgrating, with a lower reflectivity, acts as a noise-producing element

FIG. 2 shows a second embodiment, essentially an arrangement wherein thefiber Bragg gratings 6 a and 6 b of FIG. 1 have been replaced byreflectors 16 a and 16 b, which may be e.g. a set of interferometricfilters. This set of filters has essentially the same function as Bragggratings with similar reflection and transmission characteristics. Theycan either be discrete elements between the fibers (as shown in FIG. 2)or can be deposited directly onto the fiber. Also, the fiberlens 4 hasbeen replaced by another collimating means, here a lens system 14. Theoptical fiber is partitioned into three sections 15 a, 15 b, and 15 c.The light beam 7 exits from the last section of the optical fiber 15 c,as in FIG. 1. Regarding the dimensions in this second embodiment, thesame rules and calculations as described above in connection with theembodiment of FIG. 1 apply.

There is no rear reflector shown at the laser 1 in both FIGS. 1 and 2,but it is clear for a person skilled in the art that such a reflector ormirror is usually provided.

FIG. 3 depicts a layout with cavities both in front of the laser and atthe rear of it. Whereas cavities 5 a and 5 b are located essentiallysimilar to the embodiment shown in FIG. 1, some light exits the laser 1through its rear facet 22, entering, preferably through a fiberlens 24,the two rear cavities 25 a and 25 b, established by a fiber Bragggrating 26 a and a rear fiber reflector 26 b. The reflectivity at therear laser facet and/or the compound reflectivity of all rear gratingsshould be higher than 90%, preferably 100% to maximize the output powerat the laser's front facet. Again, the rules and calculations describedabove in connection with FIG. 1 apply with respect to the dimensions andreflectivities in this third embodiment,.

FIG. 4 displays a fourth embodiment of the invention with “parallel”cavities instead of a series of cavities, here a “frontal” cavity 15 aand a “lateral” cavity 33. This embodiment is shown to explain how thefunctionality of the invention can be achieved by a quasi-parallelinstead of a serial setup of feedback cavities. A beam splitter/combiner31 divides the laser beam, where typically more than 90% of the laserlight is coupled out into the fiber 15 c to exit the system, whereas acertain portion is reflected into the fiber 15 a acting as first cavity.A smaller portion of the laser light is deflected into the cavity 33 andtherein backreflected at the mirror 34. This mirror 34 ideally has areflectivity of 100%, the same as the rear laser reflector 32. Theroundtrip frequency is again determined by the lengths of the cavities,working as the necessary distortion elements leading the laser 1 tomultimode operation.

The layout shown in FIG. 4 may be modified by adding a further cavityright of the beam splitter/combiner 31 similar to the two-cavity layoutof FIG. 2. Another modification of the layout of FIG. 4 could add one ormore rear cavities similar to the layout shown in FIG. 3. To summarize,based on the teaching given and without departing from the spirit andscope of the invention, it should be relatively easy for a personskilled in the art to combine any of the designs shown, or to addportions of one design to another, and to determine the dimensionsaccording to the teaching given particularly in connection with FIG. 1.

FIG. 5 shows in graphical form the output of a laser known from therelated art whereby the bandwidth stabilization is attempted by a singlefiber Bragg grating. The vertical axis is the laser's optical bandwidthat 20 dB down from the maximum; the horizontal axis is the lasercurrent. The graph clearly shows the fluctuations by the laser'sswitching from multi-mode bandwidth (a) to a narrow single mode (b)operation or bandwidth while the laser's driving current is ramped up.In other words, the shown curve exhibits just the problem that thepresent invention tends to solve.

FIG. 6 finally exhibits the progress achieved by the present inventionin a graph similar to the one in FIG. 5. Again, the vertical axis is thelaser's optical bandwidth at −20 dB down from the maximum; thehorizontal axis is the current of a laser source according to theinvention. By use of an additional cavity formed by a second grating orother reflector system, e.g. as described in detail in connection withFIG. 1, the laser source is solely operating in multimode. There are noswitching fluctuations, the significant improvement is clearly visible.

What is claimed is:
 1. A laser source for generating a stable laser beamof a given bandwidth, including a laser and guide means for conductingthe laser beam exiting said laser's front facet, comprising a pluralityof external cavities at least partly within or as part of said laserbeam guide means, each of said cavities being established by two fixedreflectors, one of which being located in said laser beam guide means,said plurality of external cavities being dimensioned and arranged suchthat said laser operates essentially in a coherence collapse mode. 2.The laser source according to claim 1, wherein all cavities are situatedwithin the laser beam guide means, in front of the laser.
 3. The lasersource according to claim 1, wherein one or more cavities are arrangedwithin the laser beam guide means in front of the laser, and at leastone cavity is arranged at the rear of the laser.
 4. The laser sourceaccording to claim 1, including in combination a “serial” cavityarranged within the laser beam guide means, a “lateral” cavity arrangedoutside said laser beam guide means, and a beam splitter/combinerdeflecting a portion of the beam into said lateral cavity.
 5. The lasersource according to claim 4, wherein the laser emits light between 800and 1600 nm and/or the beam splitter/combiner has a reflectivity maximumwithin the bandwidth of the laser, and/or a bandwidth of itsreflectivity between 0.05 and 2 nm full-width half-maximum, and/or apeak reflectivity between 0.005 and 0.4.
 6. The laser source accordingto claim 1, wherein the reflectors located in the laser beam guide meansare Bragg gratings, whose peak wavelengths are offset and/or whosebandwidths are different.
 7. The laser source according to claim 1,wherein the laser emits light between 800 and 1600 nm and/or any of thereflectors has a reflectivity maximum within the bandwidth of the laser,and/or a bandwidth of its reflectivity between 0.05 and 2 nm full widthhalf-maximum, and/or a peak reflectivity between 0.005 and 0.4.
 8. Thelaser source according to claim 1, wherein the optical field establishedin the first cavity is out of phase with the optical field of the laser,and the optical field established in the second cavity is out of phasewith the optical field established in said first cavity, thus inhibitingphase matching with the laser and hence coherent operation of said lasersource.
 9. The laser source according to claim 1, wherein the laser is asemiconductor diode laser, and/or the laser guide means comprises anoptical fiber, either a polarization-maintaining or non-polarizationmaintaining optical fiber, and the reflectors are fiber Bragg gratingswithin said/a fiber.
 10. The laser source according to claim 1, furthercomprising means for directing the laser beam into the optical fiber, inparticular beam collimating or focusing means attached to or integratedinto an optical fiber.
 11. The laser source according to claim 1,wherein one of the fixed reflectors by which each of the cavities isestablished is the laser's front facet.
 12. The laser source accordingto claim 1, wherein the laser is an InGaAs quantum well diode laser,and/or the laser guide means comprises an optical fiber, either apolarization-maintaining or non-polarization maintaining optical fiber,and the reflectors are fiber Bragg gratings within said fiber.
 13. Amethod of making a laser source that generates a stable laser beam of agiven bandwidth, said laser source having a laser and laser beam guidemeans in front of said laser, characterized by simultaneouslymanufacturing within said laser beam guide means, a plurality of fixedreflectors, which form, together with the laser front facet, a pluralityof external cavities in front of said laser.
 14. The method of making alaser source according to claim 13, whereby the simultaneousmanufacturing is carried out by UV exposure methods creating thereflectors as fiber Bragg gratings in the optical fiber constituting thelaser beam guide means.